1. Field of the Invention
The present invention relates to a gas turbine combustor, and especially, relates to a combustor which is so constructed as to reduce drift and disturbance of airflow flowing through the interior thereof.
2. Description of the Prior Art
A cross-sectional view of FIG. 12 shows a general construction of a gas turbine. As shown in FIG. 12, a gas turbine comprises a compressor 1 compressing the air; combustors 2 being supplied with the air compressed by the compressor 1 and fuels so as to perform combustion; and a turbine 3 being rotary driven by combustion gas from the combustors 2. The compressor 1, the combustors 2 and the turbine 3 are covered by casings 4, respectively. In addition, a plurality of the combustors 2, sixteen pieces for example, are arranged on the outer circumference of a rotor 5 serving as one shaft sharing the compressor 1 and the turbine 3, being equally spaced.
In a gas turbine as described hereinabove, the air compressed by the compressor 1 is supplied to the combustors 2 and the rotor 5 through the interior of the casing 4. Then, the compressed air being supplied to the combustors 2 are used for combustion of fuels being supplied to the combustors 2. In addition, the compressed air being supplied to the interior of the casing 4 and the rotor 5 of the turbine 3 is used for cooling stationary vanes 31 fixed to the casing 4 and rotating blades 32 fixed to the rotor 5 both of which are exposed to high temperature due to combustion gas.
A combustor 2 being provided to such a gas turbine as constructed hereinabove comprises a combustor basket 2a being provided to the fuel-supply side; a transition piece 2b being connected to the combustor basket 2a and injecting combustion gas to the stationary vanes 31 in a first row of the turbine 3; and an external cylinder 2c being inserted so as to be along the inner wall of the casing 4 and covering the combustion basket 2a. Moreover, FIG. 13 shows an enlarged cross-sectional view depicting the vicinity of the combustor 2 in order to describe a detailed construction of the vicinity of the combustor 2.
As shown in FIG. 13, the combustor 2 has a combustor basket 2a comprise a pilot nozzle 21 being provided to the center thereof and performing diffusion combustion; a plurality of main nozzles 22 being provided to the outer circumference of the pilot nozzle 21, equally spaced, and performing premixed combustion; a pilot cone 23 being provided so as to cover the downstream-side tip of the pilot nozzle 21; and main burners 24 being provided so as to cover the downstream-side tips of the main nozzles 22. In addition, the compressor 1 is provided with a compressor outlet 11 for supplying the compressed air to the interior of the casing 4 where the combustor 2 is inserted, and the compressed air discharged from the compressor outlet 11 is supplied to the interior of the combustor basket 2a. 
Moreover, in order to describe the detailed construction of a combustor 2, FIG. 14 shows an enlarged cross-sectional view of a combustor basket 2a of a combustor 2. As shown in FIG. 14, a combustor 2 comprises a pilot swirl 25 being installed so as to be in contact with the outer circumference of the pilot nozzle 21 on the upstream side inside the pilot cone 23 and main swirls 26 being installed so as to be in contact with the outer circumference of the main nozzles 22 on the upstream side inside the main burners 24. As a result, the air being supplied to the pilot cone 23 is made uniform in the pilot swirl 25 and at the same time, the air being supplied to the main burners 24 is made uniform in the main swirls 26.
Additionally, the combustor 2 has a plurality of supports 27 provided to the outer circumference of the combustor basket 2a on the upstream side thereof and a rib 29 provided to support a punched metal plate 28 consisting of a perforated plate being provided to the entrance to a space between the external cylinder 2c and the combustor basket 2a on the downstream side. By having the support 27 and the rib 29 connected to the external cylinder 2c and the combustor basket 2a, the combustor basket 2a is supported and fixed to the external cylinder 2c. Furthermore, on the downstream side of the pilot nozzle 21, the main nozzles 22 are fixed by having supports 30 provided to connect the outer circumference of the pilot nozzle 21 to the main nozzles 22.
For a combustor 2 being constructed as described hereinabove, the compressed air being discharged from the compressor outlet 11 to the interior of the casing 4 flows into a space formed between the external cylinder 2c and the combustor basket 2a by way of the punched metal plate 28. The punched metal plate 28 plays a role of uniformizing the compressed air flowing into the combustor 2 by being made of a perforated plate so as to provide resistance. The compressed air flowing into the space between the external cylinder 2c and the combustor basket 2a by way of the punched metal plate 28 flows along the inner wall of the external cylinder 2c. 
In consequence, by having the compressed air make 180 degrees turn at the bottom part of the external cylinder 2c (the bases of the pilot nozzle 21 and the main nozzles 22), the compressed air flows in between the supports 27 supporting the combustor basket 2a and is supplied to the interior of the combustor basket 2a. Then, finally, a swirling flow is supplied by the pilot swirl 25 and the main swirls 26 of the combustor 2 so as to be used for diffusion combustion by the pilot nozzle 21 and used for premixed combustion by the main nozzles 22.
However, the compressed air being supplied to the combustor 2 in a manner as described hereinabove becomes unstable in flowing inside the combustor 2. To be specifically, a vortex flow due to flow separation is generated on the inner wall of the combustor basket 2a serving as the outside of the main nozzles 22, and a vortex flow due to turning of the flow of the compressed air is generated on the base of the pilot nozzle 21, respectively. In addition, a vortex flow flowing along the pilot nozzle 21, a vortex flow flowing along the inner wall of the combustor basket 2a toward the outlet of the combustor basket 2a and the like are also generated. Due to these vortex flows, the flow of the compressed air inside the combustor basket 2 becomes unstable.
As a result, pressure distribution of the compressed air at the tips of the pilot nozzle 21 and the main nozzles 22 becomes imbalanced, resulting in unstable combustion thereof. In consequence, not only the rate of occurrence of NOx becomes high but also durability becomes deteriorated due to generation of combustion oscillations. For these disadvantages, the present applicant proposes a combustor in which disturbance and drift of the compressed air flow are restrained by installing a flow ring having a semicircular cross-sectional configuration and a ring shape to a position where the supports 27 are connected to the combustor basket 2a. (See the Japanese Patent Application Laid-Open No. 2000-346361.)
In the Japanese Patent Application Laid-Open No. 2000-346361, it is disclosed that by installing a punched metal plate between the outside of the pilot nozzle 21 and the inside of the main nozzles 22 so as to provide resistance, uniformity of the compressed air flow flowing inside the combustor basket 2a is maintained. In addition, it is disclosed that by installing guide vanes nearer to the base side of the pilot nozzle 21 than a flow ring, the uniformity of the flow is maintained when the compressed air turns 180 degrees.
Being constructed as described hereinabove, it is possible to relatively stabilize the compressed air flow which flows, making 180 degrees turn inside the combustor basket 2a. However, because a difference occurs between the inside compressed air flow and the outside compressed air flow in turning, the uniformity thereof is not sufficient. As a result, it is necessary to compensate the uniformity by lengthening the distance from the position of the flow ring to the tip of the pilot nozzle 21 and the distance from the position of the flow ring to the main nozzles 22, respectively.
Therefore, not only the combustor 2 needs to be enlarged but also supports 30 are necessary to fix the tips of the main nozzles 22, but the supports 30 disturb the flow of the compressed air. To be specific, the supports 30 fixing the tips of the main nozzles 22 contribute to deterioration of uniformity of the flow of the compressed air inside the combustor 2. In addition, not only because the supports 30 give an adverse effect to uniformity of the compressed air flow inside the combustor 2 but also because the bottom portion (back surface side) of the external cylinder 2c is not constructed for purpose of turning at the position where the compressed air turns 180 degrees, instability of the flow in the external cylinder 2c is not eliminated. Moreover, there arises a problem of a significant pressure drop because resistance based on bending of the flow ring and guide vanes is significant.